1. Field of the Invention
The present invention relates to a method of regenerating a carbon dioxide gas absorbent, in particular, a method of regenerating a carbon dioxide gas absorbent using lithium silicate as a carbon dioxide gas absorbent.
2. Description of the Related Art
In combustion apparatuses such as engines, which burn fuels mainly consisting of hydrocarbon, carbon dioxide gas is removed in the vicinity of a combustion chamber having a high carbon dioxide gas density, and thereby carbon dioxide gas in an exhaust gas can be efficiently removed. The vicinity of the combustion chamber having a high carbon dioxide gas density has a temperature of 300° C. or more, and thus it is required to absorb carbon dioxide gas under the temperature circumstances.
As well-known methods of separating carbon dioxide gas, there are a method using cellulose acetate, and a chemical absorption method using an alkanolamine solvent. However, in these separating methods, it is necessary to restrict a temperature of an introduction gas to about 200° C. or less. Therefore, to apply the separation methods to exhaust gases that requires removing at a high temperature, such as carbon dioxide gas exhausted from combustion apparatuses, it is necessary to cool the exhaust gas by a heat exchanger or a cooling tower, etc. This increases an energy consumption amount necessary for separating the carbon dioxide gas.
In such methods of separating carbon dioxide gas, Jpn. Pat. Appln. KOKAI Pub. No. 9-99214 discloses using a carbon dioxide gas absorbent containing lithium zirconate, and Jpn. Pat. Appln. KOKAI Pub. Nos. 2000-262890 and 2001-170480 disclose using a carbon dioxide gas absorbent containing lithium silicate. The carbon dioxide gas absorbents containing lithium zirconate or lithium silicate can absorb carbon dioxide gas at a temperature region exceeding 500° C. On the other hand, the absorbents release the carbon dioxide gas when the temperature reaches 750° C. or more. Further, the documents disclose that adding alkaline carbonate selected from lithium, sodium and potassium promotes absorption reaction of the carbon dioxide gas, and enables efficient absorption of carbon dioxide gas of a low concentration.
However, adding alkaline carbonate increases time required for releasing the absorbed carbon dioxide gas. Besides, in this case, the carbon dioxide gas is released at a higher temperature. Therefore, it is difficult for carbon dioxide gas absorbents containing alkaline carbonate to simultaneously satisfy properties required for absorption and release of carbon dioxide gas.
In comparison with this, Jpn. Pat. Appln. KOKAI Pub. No. 2001-299131 discloses that carbon dioxide gas absorbent containing alkaline carbonate with an amount of 0.5 to 4.9 mol % can release gas at a lower temperature. However, the temperature of releasing carbon dioxide gas is 750° C. or more due to thermodynamic equilibrium. This further densification of the carbon dioxide gas absorbent, and causes deterioration of the absorbent in the end.